A classifying wheel of the above-mentioned type has been known e.g. by U. S. Pat. No. 4,786,406. There, an air classifier is described whose classifying wheel comprises blades entirely made of a wear-resistant sintered material and inserted with axial clearance into recesses of a circular disc carrying the classifying wheel hub on the one hand and of an annular-like cover disc on the other hand. The blades are fixed in their mutual position and radially supported against the circular and the cover disc by inserts matching the recesses and made of a rubber-elastic, wear-resistant material. Materials of highest purity whose products are made useless even with the slightest contamination due to e.g. abrasion at product-contact surfaces can be processed free of wear and abrasion by means of this air classifier type. Even with highly abrasive materials of a Mohs' hardness of above 4, wear-free and thus profitable operation of the air classifier is possible.
This classifier, however, is adversely affected by the efforts required for dynamic balancing of the classifying wheel composed from various parts, as the classifying wheels are operated at maximum peripheral speeds of above 50 m/s. Furthermore, classifying wheels equipped with blades made of sintered material are not as rigid as those made of steel and thus can be operated only at approx. 80% of the maximum peripheral speed of the steel classifying wheels without modification of the sealing between classifying wheel and suction tube for the fines.
From GB 20 61 399 A it has been known that the single vanes of a pump impeller together with parts of the cover discs axially limiting the flow channels are shaped as one-piece vane element made of a refractory, wear-resistant material (mentioned are tungsten carbide or aluminium oxide) by metal-cutting processing before sintering. The single vane elements are put together after sintering to be the impeller, and this one is mounted between two shaped metal discs, resulting in a sandwich construction of the required strength. The sintered-material parts exclusively serve for wear protection against the conveyed fluid containing solids. In this process merely sliding friction takes place, in that the geometry of the vane channels, as with all flow machines is designed for utmost wall-parallel fluid flow.
With centrifugal-wheel air classifiers of the type considered, completely different flow conditions in the vane channels are to be observed. It is true, that corresponding to a pump or blower a blade wheel is there present as an essential operational component, but a blower outside the air classifier produces a flow in the blade channels of the classifying wheel contrary to the flow produced by operation of the classifying wheel as blower impeller, i.e., the flow through the classifying wheel passes from its external periphery radially to the inside, and thus against its centrifugal action. As a result, sharp flow deflection develops at the blade edges at the wheel periphery giving a centrifugal acceleration to the product particles conveyed by the classifying air as one factor that controls the classification process.
This then results from the combined effect of the centrifugal force of the particles and the sweeping force applied on the particles by the classifying air. With smaller particles, the sweeping force predominates, thus they are taken by the classifying air and discharged as fines.
With bigger particles and agglomerates of smaller particles, the centrifugal force predominates, thus these particles receive a movement at right angles to flow direction of the classifying air which deflects them outwards and causes impacting with the blades of the classifying wheel. Compact particles are therefore thrown outwards and discharged as coarse material. Agglomerates are disintegrated into the individual particles and re-classified. Contrary to pumps or blowers, the blades of the classifying wheel are mainly stressed by impacting forces and the resulting wear.
This type of stress can be completely controlled by means of the classifying wheel described by U.S. Pat. No. 4,786,406 disregarding the mentioned disadvantages.